1. Field of the Invention
The present invention relates to a method for determining the gas content in a breathing apparatus as well as to a breathing apparatus operating according to the method.
2. Description of the Prior Art
Accurate regulation of the oxygen content of breathing gas supplied to a patient with a breathing device is vitally important. Oxygen content is measured by an oxygen meter to ensure that the delivered oxygen content is correct. If the oxygen content of expired gas is also measured, the patient""s consumption of oxygen can be established. Important information can also be derived from the amount of expired carbon dioxide. In other contexts, determination and monitoring of the concentration of other gases supplied, such as helium, also may be desirable.
One known method for determining the content of a gas component in a binary gas mixture (e.g. the oxygen content of a mixture of air and oxygen) involves determination of the speed of sound in the mixture. The speed of sound is usually measured with ultrasound, however, this type of measurement is temperature-related, since the speed of sound is temperature-related. The temperature of the mixture therefore is usually measured as well (or the temperature in a test chamber and test sample is regulated in such a way that a known temperature is maintained.)
Temperature variation is one problem encountered in the use of an ultrasonic-type gas meter in breathing apparatuses. The oxygen content of delivered breathing gas should be measured in every respiratory cycle. During inspiration, gas flows through the inspiration line at high speed for a relatively brief period. Temperature variations are therefore very striking and pose difficulties in simultaneous determination of temperature and air speed in an accurate and reliable manner.
An object of the present invention is to provide a method for determining the gas content of breathing gas in breathing devices which avoids the aforesaid problems.
Another object of the invention is to provide a breathing apparatus in which correct measurement of the gas content of breathing gas can be made.
The above object is achieved in accordance with the principles of the present invention in a method for determining a gas content in a breathing apparatus, and in a breathing apparatus operating according to the method, wherein the gas content is determined from the speed of sound in the gas, and wherein the determination of the speed of sound is synchronized with one or more specific times in a respiratory cycle.
By synchronizing measurements of the speed of sound with specific times in respiratory cycles, measurement can be made when the temperature is stable and easy to measure/monitor.
Performing the measurement immediately before an inspiration commences is particularly advantageous in the determination of the gas content of breathing gas for delivery to a patient. Conditions in the inspiration section of the breathing apparatus are then most stable. The fact that this causes a de facto one-breath lag in measurement of e.g. oxygen content does not pose any risk to the patient. At worst, she/he only receives one breath with a (partially) erroneous gas composition.
In a corresponding manner, the gas content in expired air can be determined by measurement before an expiration phase begins. Measurement of the content of both inspired and expired gas supplies information on the patient""s uptake. Compensation must be made in this measurement, however, for the patient""s contribution of carbon dioxide to the gas mixture. This is easily achieved with a carbon dioxide meter installed next to the oxygen meter.
Measurement can be made by pulsing ultrasound at a certain clock frequency in order obtain a series of measurement values at every determination of oxygen content. The measurement values can be used for identifying a trend.
If the trend is stable (the same results are obtained at every measurement point), the determined oxygen content can be accepted as correct.
If the trend is unstable, i.e. measurement values vary, the oxygen content can still be estimated by analyzing the measurement values. Exponential regression is a known method for such an analysis. It yields a sufficiently accurate approximation of oxygen content.
In an embodiment of the apparatus, the oxygen meter incorporates a measurement chamber with an inlet and outlet connected to an inspiration line. The inlet and outlet are equipped with sintered filters, causing heat to be stored, so the temperature in the measurement chamber is kept more constant. A slight drop in pressure, large enough to divert part of the flow during inspiration into the measurement chamber to replace the gas sample, is created with the aid of a mesh in the inspiration line.
In an alternative embodiment, valves are placed at the inlet and the outlet to control the exchange of gas samples.